Increase in the concentrations of amino acids in the vascular tissue of white clover and white lupin after defoliation: an indication of a N feedbackregulation of symbiotic N-2 fixation

Defoliation, particularly experienced by pasture and forage legumes, is ass ociated with a fast and dramatic decrease in the nitrogenase activity (C2H2 reduction, H-2 evolution in Ar/O-2). Since the plant's N demand is tempora rily reduced after defoliation, the nitrogenase activity may be down-regula ted through a N feedback mechanism. To test this hypothesis, the concentrat ions of free amino acids in the xylem of white clover and in the phloem of white lupin were measured before and after defoliation. The concentrations of free asparagine, glutamine and aspartate were the highest of all the ami no acids before defoliation. The concentrations of these amino acids increa sed three-fold in the xylem of white clover and two-fold in the phloem of w hite lupin within one to three hours after defoliation. The results are con sistent with the concept of a N feedback mechanism that regulates nitrogen fixation in legume plants, in which free amino acids, cycling in the vascul ar tissue, act as trigger molecules.